1. Field of the Invention
The present invention relates to a toner for developing electrostatic latent images and an image-forming apparatus, more particularly, a toner for developing electrostatic latent images which is used in an image-forming apparatus such as a copying machine and an apparatus for forming images using the toner.
2. Description of Related Art
Highly charge-receptive toners have been proposed (for example, Japanese Unexamined Patent Publication No. HEI 5(1993)-72805) which exhibit a good triboelectrification property and are not liable to form blurs in print and spots in non-printed part, not only in developing apparatuses used by a two-component developing method which is a dry development method but also in developing apparatuses used by a one-component developing method in which toners are not in frequent contact with charge donor materials and in developing apparatus in which charge donor materials have a poor charge donating efficiency.
The toner of the above-mentioned publication is comprised of a polyester resin, a colorant and an electric charge controlling agent. As the polyester resin, used is one having an acid value not greater than 15 mgKOH/g, and as the electric charge controlling agent, used is a chromium complex compound represented by the following formula: ##STR1##
(wherein X is Cl, Br, SO.sub.2 NH.sub.2, SO.sub.2 CH.sub.3 or SO.sub.2 C.sub.2 H.sub.5, and A.sup.+ is a C.sub.8-16 straight-chain alkylammonium or a C.sub.8-16 branched alkylammonium in which the alkyl moiety is optionally interrupted by an oxygen atom.
The reason why the acid value of the polyester resin in the above toner is 15 mgKOH/g or less is that, if the acid value is over 15 mgKOH/g, free carboxyl groups contained in the polyester resin, which have electron receptivity, improve negative electrification of the toner itself, while chelated rings of a chromium complex compound become liable to decompose. Therefore, the chromium complex compound is prevented from displaying its function as an electric charge controlling agent sufficiently. Especially, if the acid value exceeds 25 mgKOH/g, time constant until the charged triboelectrified amount of the toner reaches its saturation becomes large, and therefore, sufficient triboelectrification is difficult to obtain in the developing apparatus.
On the other hand, if the acid value is 10 mgKOH/g or below, the chromium complex compound is not decomposed at chelate rings due to the effect of carboxyl groups, which brings extremely good electrification characteristics by synergism with readiness to negative electrification that the polyester resin has.
However, if the acid value of the polyester resin is 15 mgKOH/g or below, excellent negative electrification and offset-resistant properties intrinsic to the polyester resin are restricted. A problem lies in that the polyester resin cannot exhibit its excellent properties fully.